Timers for automatic switching have been used extensively in copy machines. Such timers have been used to automatically discontinue the operation of various devices within predetermined times after manually starting or after a malfunction occurs in the machine. Some such devices controlled by timers in copy machines include pumps, motors, and other components. The operation of these devices is automatically terminated by a timer after predetermined time intervals or because of faulty operation.
Copy machines used heretofore of the type involving the present invention include a liquid electrostatic developer and a rotatable drum having a photoconductive surface. The surface of the drum is first moved past a charging station and an image to be copied is projected onto the surface of the drum at an exposure station forming a latent electrostatic image thereon. The drum is then rotated through a reservoir which contains a liquid developer having charged toner particles suspended in a suitable carrier liquid or dispersant. After rotation through the reservoir the drum has a developed electrostatic image on its surface produced as a result of the toner particles being attracted to the latent image on the drum. Next, the drum is brought in contact with paper where the toner is transferred from the surface of the drum onto the paper. Electrostatic liquid developer systems of the type described have been disclosed and described in greater detail in numerous patents and publications.
One of the problems with this type of copy machine involves the liquid developer through which the drum passes. The liquid developer, as mentioned above, includes a dispersant or liquid carrier and charged toner particles in solution. A pump, actuated by an operator closing a power-on switch, is employed to circulate the developer prior to making copies to assure that the toner particles are suitably mixed with the dispersant.
When the copy machine is not operated over relatively long periods of time, the liquid developer is not circulated and the toner particles tend to settle out of solution, i.e., separate from the dispersant. The surfaces of the pump, tubes, and drum in the copy machine in contact with the liquid developer, become coated with grit because of the build-up of the separated toner particles. In addition, the toner particles in the reservoir tend to settle to the bottom forming a thick, sticky residue. The creation of grit particles on the drum surfaces results in poor quality copies and the build-up of separated toner particles on the surfaces of the pump and tubes and bottom of the reservoir requires expensive and time consuming cleaning operations. It has been found in practice, that this problem generally develops during long periods of non-use of the copy machine, such as during weekends and holidays in places of business where such machines are used.
Another aspect of the problem associated with the non-circulation of the liquid developer over long periods of time results from the lack of immersion of a portion of the surface area of the drum in reservoir containing the liquid developer. The exposed portion of the drum surface tends to build up a coating of particles because it is not immersed in the carrier liquid. After a prolonged period of exposure the grit build-up on this portion is not readily dissolved during subsequent immersion in the liquid carrier.